The invention is based on a method for monitoring a cooling fluid circuit of an internal combustion engine.
In current reciprocating piston internal combustion engines for motor vehicles, the heat transmitted from a wall of a combustion chamber to a cylinder head and cylinder block is essentially dissipated by means of a cooling fluid. This fluid is circulated in a cooling fluid circuit by a pump, which is as a rule mechanically driven by the internal combustion engine. There are also designs in which a controllable electric motor is used as a pump drive unit in order to produce a cooling as needed.
A regulating valve distributes the volumetric flow of the cooling fluid to a radiator and a bypass line, which is provided parallel to the radiator. In addition to the radiator, a heat exchanger for the passenger compartment is also connected to the cooling fluid circuit. A desired value for the temperature of the cooling fluid, possibly controlled by means of a characteristic field, is set so that the permissible temperatures of the components to be cooled and of the cooling fluid are never exceeded during operation.
DE 41 09 498 A1 has disclosed a device and a method for a very sensitive regulation of the temperature of an internal combustion engine. To this end, a control device is supplied with a number of input signals, e.g. the temperature of the engine, the speed and load of the engine, the vehicle speed, the operating state of an air conditioning system or heater of the motor vehicle, and the temperature of the cooling water. A desired value transmitter of the control unit takes the input signals into account when establishing a desired value for the temperature of the engine. According to a comparison of the actual values to the desired values, the control unit controls a three-way valve, which is disposed in the mouth region of a bypass line in a line between the engine and a radiator.
Depending on the position of the three-way valve, the inlet flow is distributed to the cooling circuit and the bypass line. Therefore a cooling of the engine is detected not only as a function of operating parameters that are of direct significance to the temperature build-up, but also as a function of parameters of additional units, which only influence the temperature indirectly. Furthermore, the possibilities for setting the optimal temperature are broadened significantly since malfunctions can also be detected and taken into account. The association of different operating conditions with different ranges of desired values of the temperature permits the desired temperature to be quickly adjusted, which can be further refined through different priorities of the operating conditions.
In order to detect a defect in the cooling circuit, e.g. a defect in a temperature sensor, there are a variety of strategies, for example short circuit detection or plausibility analyses using a modeled comparison temperature. In this modeling, the measured, possibly distorted, temperature of the engine is compared to a model that depends on the energy throughput. If the measured temperature falls below the modeled temperature by a definite amount, then an error is stored and displayed.
If the cooling fluid is partially frozen due to insufficient antifreeze, without any damage as yet to the engine, then the control unit detects a nearly constant temperature below the freezing point until the cooling water has thawed. It can take up to 200 seconds for the cooling water to thaw. But since the model temperature presupposes that after a certain delay, the temperature of the cooling fluid increases, after a short time, an error is stored, which indicates a defective sensor or defective wiring. But the actual cause is the lack of antifreeze in the cooling fluid.